Manage your subscription

Cuban flowers rely on bat signal for survival

By Cian O'Luanaigh

Gotham City famously relies on a bat signal to call for help. In the rainforests of Cuba grows a flower with its own version – a device that is vital for the flower’s survival.

The rainforest vine Marcgravia evenia has an unusual anatomy. It has a disc-shaped leaf suspended above a ring of flowers, below which hang cup-shaped nectaries. In cross section, the disc leaf looks like the reflector of a satellite dish&colon; the plant’s other leaves are much flatter in comparison.

“When I saw a picture of this plant I suspected this might be an adaptation,” says Ralph Simon of the University of Ulm in Germany. “We saw the bats were crazy about the flowers, licking the nectar.”

Using a loudspeaker-microphone combination, Simon and colleagues played sounds at the plant and analysed the resulting echoes. They found striking differences between the acoustic properties of disc-shaped leaves and the flatter ordinary leaves.

Advertisement

Ordinary leaves produced a strong echo when face-on to the loudspeaker-microphone, but the signal strength dropped – and its acoustic signature changed – as the loudspeaker-microphone swung round to a position edge-on to the leaf. The signal from a disc-shaped leaf remained strong and characteristic no matter where the loudspeaker-microphone was placed within an 80-degree arc in front of the leaf.

Echo guide

Simon reckons the predictable echo signal of the disc-shaped leaf might help bats find Marcgravia flowers against the random and variable echo background of the rainforest.

“The disc leaf has a signature which is constant from many different directions – from every direction you get the same echo,” says Simon. “That’s very conspicuous to a bat.”

To test this, he trained three male nectar-feeding bats (Glossophaga soricina) to search for a feeder 2.5 centimetres wide – roughly the same size as a single nectary – in a chamber filled with artificial foliage.

The time it took the bats to find the feeder halved when a replica of the disc-shaped leaf was fastened above it. Attaching a replica of an ordinary leaf instead made no difference to the search time.

Though the disc shape makes the leaf less efficient at photosynthesis, Simon says that the benefits of attracting bats outweigh this energetic cost&colon; bats are good pollinators, and their large home range means they can take pollen to plants far apart from each other, and so help their reproduction, he says.

Cleared to pollinate

“An ultrasonic beacon is an astonishingly effective thing – it’s the same principle that airports use to guide aircraft in to land,” says Brock Fenton of the University of Western Ontario in London, Canada, who was not involved in the study.

Given there are several hundred species of bat-pollinated flower in Central and South America, and 40 bat species highly adapted as nectar feeders, Simon’s team expects to find more strangely shaped bat signals on plants.

“The plants are obviously exploiting the bat,” says Fenton. “There are all these little jewels waiting out there for someone to pursue.”